Added reason to WebSocket Disconnect

Clients can send this reason field when disconnecting. This is allowed as per RFC 6455 and supported in browsers JS API, but previously not supported in
ASGI.

Fix #459

asgiref/typing.py

@@ -189,6 +189,7 @@ class WebSocketResponseBodyEvent(TypedDict):
 class WebSocketDisconnectEvent(TypedDict):
  type: Literal["websocket.disconnect"]
  code: int
+ reason: Optional[str]
 
 
 class WebSocketCloseEvent(TypedDict):

specs/www.rst

@@ -22,6 +22,7 @@ This spec has had the following versions:
 * ``2.2``: Allow ``None`` in the second item of ``server`` scope value.
 * ``2.3``: Added the ``reason`` key to the WebSocket close event.
 * ``2.4``: Calling ``send()`` on a closed connection should raise an error
+* ``2.5``: Added the ``reason`` key to the WebSocket disconnect event.
 
 Spec versions let you understand what the server you are using understands. If
 a server tells you it only supports version ``2.0`` of this spec, then
@@ -457,6 +458,10 @@ Keys:
  was received in the frame from the client, the server should set this to ``1005``
  (the default value in the WebSocket specification).
 
+* ``reason`` (*Unicode string*) -- A reason given for the disconnect, can
+ be any string. Optional; if missing or ``None`` default is empty
+ string.
+
 
 Disconnected Client - ``send`` exception
 ''''''''''''''''''''''''''''''''''''''''